Fluids, especially lubricating oils or the like, are exposed to a continuous ingress of contaminants in the form of ferromagnetic particles, other metallic particles or other particles of contaminants in the operation of systems or devices. A certain maximum concentration of contaminants, depending on the application, can be tolerated for the operation of a system or of devices. Especially for rolling oils, as are used in the steel industry or heavy industry, under certain circumstances large amounts of contaminants are introduced. Their content can hardly be determined with sensors according to the prior art.
EP 290 397 B1 discloses a sensor for detecting the content of ferromagnetic particles in a fluid having a primary magnetic circuit formed by a permanent magnet. The primary magnetic circuit has an air gap which can be exposed to the fluid to cause the particles to collect in the vicinity of the air gap. The sensor also has a Hall element which detects the change of the magnetic flux depending on the collected contaminants. A proportional detector signal is achieved on the content of particles of contaminants in the fluid. A secondary magnetic circuit with an induction coil is designed to counteract the primary magnetic circuit of the permanent magnet up to a complete extinction of the magnetic field in the air gap. In this way, detachment of the particles collected in the air gap is enabled. The known sensors and measuring methods for determining the contamination of a fluid are, however, inherently unsuited to a substantial ingress of contaminants or ingress of metallic particles.